/Engine/Plugins/Experimental/MeshModelingToolsetExp/Source/MeshModelingToolsEditorOnlyExp/Public/BspConversionTool.h
The more complicated conversion path, where we convert brushes individually and use static mesh boolean operations to combine them. Boolean operations fail when geometry is non-manifold, which happens in the case of stair brushes, mainly, due to them not being properly closed. The preview gets cleared in the case of an error.
false if BrushesToConvert is empty or a conversion gives invalid geometry, true otherwise.
bool ConvertThenCombine ( [FText](API\Runtime\Core\Internationalization\FText) * ErrorMessage )